Articulated electric propulsion system with fully stowing blades and lightweight vertical take-off and landing aircraft using same

ABSTRACT

An aerial vehicle adapted for vertical takeoff and landing using pivoting thrust producing elements for takeoff and landing. An aerial vehicle which is adapted to takeoff with thrust units providing vertical thrust and then transitioning to a horizontal flight path. An aerial vehicle with pivoting thrust units with propellers, wherein some or all of the propellers are able to be stowed and fully nested during forward flight.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/218,845 to Bevirt et al., filed Mar. 18, 2014, which ishereby incorporated by reference in its entirety.

BACKGROUND

Field of the Invention

This invention relates to powered flight, and more specifically to avertical take-off and flight control aircraft and flight method.

Description of Related Art

There are generally three types of vertical takeoff and landing (VTOL)configurations: wing type configurations having a fuselage withrotatable wings and engines or fixed wings with vectored thrust enginesfor vertical and horizontal translational flight; helicopter typeconfiguration having a fuselage with a rotor mounted above whichprovides lift and thrust; and ducted type configurations having afuselage with a ducted rotor system which provides translational flightas well as vertical takeoff and landing capabilities.

With VTOL aircraft, significantly more thrust may be required fortakeoff and landing operations than during regular forward flight. Thisextra thrust may also be required during the transitions betweenvertical and horizontal flight. In the case of propeller drivenaircraft, for example, with a plurality of pivoting thrust units usingpropellers for takeoff, some or many of these thrust units may be idledduring regular, horizontal forward flight.

What is called for is a thrust unit utilizing a propeller which allowsfor rotation of the thrust unit from a position of vertical thrust to aposition wherein the thrust unit provides horizontal thrust. What isalso called for is a thrust unit which is capable of stowing thepropeller blades completely, into a nested configuration.

SUMMARY

An aerial vehicle adapted for vertical takeoff and landing usingpivoting thrust producing elements for takeoff and landing. An aerialvehicle which is adapted to takeoff with thrust units providing verticalthrust and then transitioning to a horizontal flight path. An aerialvehicle with pivoting thrust units with propellers, wherein some or allof the propellers are able to be stowed and fully nested during forwardflight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an aerial vehicle in a takeoffconfiguration according to some embodiments of the present invention.

FIG. 2 is a perspective view of an aerial vehicle in a forward flightconfiguration according to some embodiments of the present invention.

FIG. 3 is a view of a stowing blade system in a deployed forward flightconfiguration according to some embodiments of the present invention.

FIG. 4 is a perspective view of a stowing blade system in a stowedconfiguration according to some embodiments of the present invention.

FIG. 5 is a front view of a stowing blade system in a stowedconfiguration according to some embodiments of the present invention.

FIG. 6 is a partial view of a stowing blade system in a stowedconfiguration according to some embodiments of the present invention.

FIG. 7 is a front partial view of a stowing blade system in a stowedforward flight configuration according to some embodiments of thepresent invention.

FIG. 8 is a partial view of a stowing blade system in a stowedconfiguration according to some embodiments of the present invention.

FIG. 8A is an illustration of a fin mount according to some embodimentsof the present invention.

FIG. 9 is a partial view of a stowing blade system in a stowedconfiguration according to some embodiments of the present invention.

FIG. 10 is a side view of an exemplary blade stowed according to someembodiments of the present invention.

FIG. 11 is a side view of an articulated mounting system in a forwardflight configuration according to some embodiments of the presentinvention.

FIG. 12 is a side view of an articulated mounting system in a take offconfiguration according to some embodiments of the present invention.

FIG. 13 is a side view of an articulated mounting system in atransitioning configuration according to some embodiments of the presentinvention.

FIG. 14 is a top view of an articulated mounting system in atransitioning configuration according to some embodiments of the presentinvention.

FIG. 15 is a perspective view of an articulated mounting system in atransitioning configuration according to some embodiments of the presentinvention.

FIG. 16 is a partial side view of an articulating mounting system withits blades deployed according to some embodiments of the presentinvention.

FIG. 17 is a rear perspective view of an articulated mounting systemaccording to some embodiments of the present invention.

FIG. 18 is a partial view of the underside of a rotor hub according tosome embodiments of the present invention.

FIG. 19 is a partial side cutaway view of the stowing mechanicsaccording to some embodiments of the present invention.

FIG. 20 is a bottom perspective view of the rotor stowing mechanicsaccording to some embodiments of the present invention.

DETAILED DESCRIPTION

Although vertical takeoff and landing (VTOL) aircraft have always beendesired, compromises in the realization of these aircraft have limitedtheir usefulness and adoption to certain niches. The thrust needed forVTOL is significantly higher than the thrust needed to maintainhorizontal flight. The vertical take-off thrust may also be neededduring the transition to forward flight. Once moving in forward flight,the wings of the aircraft provide lift, supplanting a function deliveredby motors during VTOL and during transition. Thrust producing elementsneeded during take-off, but not during forward flight, may be alteredduring forward flight such that they impart less drag onto the flyingsystem.

In some aspects, an aerial vehicle may use bladed propellers powered byelectric motors to provide thrust during take-off. The propeller/motorunits may be referred to as rotor assemblies. In some aspects, the motordriven propeller units on the wings may rotate relative to a fixed wing,such that the propellers provide vertical thrust for take-off andlanding. The rotation of the motor driven propeller units may allow fordirectional change of thrust by rotating both the propeller and theelectric motor, thus not requiring any gimbaling, or other method, oftorque drive around or through a rotating joint. The motor drivenpropeller units may be referred to herein as motor driven rotor units.

In some aspects, some or all of the wing mounted motor driven rotors areadapted to have the rotor blades fold back into a stowed positionwherein the blades nest in recesses in the adjoining nacelle body aftera transition to horizontal flight. The nested blades may result in asignificantly lower drag of the aerial vehicle, while also allowing asignificantly reduced power usage with only some of the rotors providingforward thrust.

In some aspects, extended nacelles with two coaxial propellers are usedsuch that one of the propellers is used during forward flight, andanother during vertical take-off and landing. The VTOL propeller may beadapted to nest its blades during forward flight. In some aspects, theextended nacelle may reside at the tip of a wing, or at the end of arear V-tail element. In some aspects, each of the coaxial propellers hasits own electric motor. In some aspects, the coaxial propellers aredriven by the same electric motor. In some aspects, the electric motorhas directional clutches such that one propeller is driven while themotor rotates in a first direction, and the other propeller is drivenwhile the motor rotates in a second direction.

In some aspects, the mass balance of the aerial vehicle may be alteredby movement of masses such as the battery mass. In some aspects, thebattery mass may be adjusted to retain balance when a different numberof occupants are supported. In some aspects, mass balance may beadjusted in automatic response to sensors within the aerial vehicle. Insome aspects, the battery mass may be distributed between a two or morebattery packs. The battery packs may be mounted such that their positionmay be changed during flight in response to changes in the balance ofthe aerial vehicle. In some aspects, the flight control system of theaerial vehicle may sense differential thrust requirements duringvertical take-off and landing, and may move the battery mass in order toachieve a more balanced thrust distribution across the rotor assemblies.In some aspects, the battery mass may be moved should there be a failureof a rotor assembly during transition or vertical take-off and landing,again to balance the thrust demands of the various remaining functioningrotors.

In some embodiments of the present invention, as seen in FIG. 1, anaerial vehicle 100 is seen in take off configuration. The aircraft body101 supports a left wing 102 and a right wing 103. Motor driven rotorunits 140 include propellers 107 which may stow and nest into thenacelle body 106. The aircraft body 101 extends rearward is alsoattached to raised rear stabilizers 104. The rear stabilizers have rearmotors 105 attached thereto. Portions of the rotor unit have beenomitted in FIG. 1 for illustrative clarity.

FIG. 1 illustrates the aerial vehicle 100 in a vertical take-off andlanding configuration such that the thrust of the rotors is directedupward. The propellers 107 have been rotated relative to the nacellebodies 106 using articulated linkages. In this vertical take-off andlanding configuration, the aerial vehicle 100 is able to utilize sixpropellers providing thrust in a vertical direction. The propellers 107are adapted to raise the vehicle 100. After the initial verticaltake-off, the vehicle transitions to forward horizontal flight. Thetransition is facilitated by the articulation of the propellers from avertical thrust configuration to positions off of vertical,transitioning to a horizontal thrust configuration. FIG. 3 isillustrative of the motor driven rotor unit in a powered forward flightconfiguration.

As the aerial vehicle 100 transitions to a forward, horizontal, flightconfiguration, the wings 102, 103 begin to provide lift. Once travelingin a horizontal attitude, with speed, significantly less thrust isneeded to propel the aerial vehicle 100 forward than was needed asvertical thrust during take-off. FIG. 2 illustrates a forward flightconfiguration of an aerial vehicle 100 wherein the blades 108 of thepropellers 107 have been stowed into recesses 110 on the nacelle bodies106. With the blades stowed during forward flight, a low drag profilemay be attained. In some aspects, some of the main propellers 107 may beused for forward flight. In some aspects, all of the main propellers 107may be stowed, and alternate forward flight propellers 111 may be usedin forward flight.

In an exemplary configuration of the first embodiment, the aerialvehicle has 6 rotors and weighs 900 kg. The rotor diameters are 2.1meters, with a thrust per rotor of 1500 N in hover. The continuous rpmof the motor at sea level is 1030 rpm, with a maximum of 1500 rpm. Thewingspan is 7.5 meters. The battery mass is 360 kg, and the mass permotor is 9 kg. The cruise speed is 320 km/h. The continuous hover shaftpower per motor is 25 kW at standard sea level conditions.

FIGS. 3 and 4 illustrate the deployed and stowed configurations,respectively, of the main propellers 107 of the motor driven rotor units140. In the deployed configuration, the propeller blades 108 of thepropeller 107 are deployed to a position approximately perpendicular tothe rotation axis of the motor driven rotor unit 140. The actual bladeangle may vary as a function of motor rpm and other factors, asdiscussed below. A spinner 109 presents a leading surface for the motordriven rotor unit 140.

In the stowed configuration, the blades 108 reside within recesses 110in the nacelle body 106. As seen in front view in FIG. 5, in the stowedconfiguration the outer surface of the forward portion of the nacelle iscomposed of the surfaces of the blades 108 of the propeller 107. Theouter surface of the nacelle with the blades in the stowed configurationis a composite of the five blades' surfaces. The blades and the nacellesmay be designed in concert such that the nacelle aerodynamicrequirements and those of the propeller fit into each other into acomplementary design. The recesses 110 may be adapted to provide a verysnug fit for the blades 108 in the stowed configuration.

FIGS. 6 and 7 illustrate a perspective view and a front view,respectively, of a motor driven rotor unit with the spinner removed tohelp the viewer visualize a design according to some aspects of thepresent invention. The main hub 122 is seen as a mounting point for eachof the five propeller blades 108. The main hub 122 provides the mainsupport of the propeller blades, which are each pivotally connected tothe main hub. The main hub 122 also provides the drive torque to theblades 108 of the propeller 107. As discussed further below, the mainhub 122 is coupled to the outboard bracket of the rotor deploymentmechanism via a rotary bearing, or bearing assembly.

FIG. 8 illustrates a perspective view of a motor driven rotor unit withfurther portions removed for clarity of illustration. The propellerblade 108 is illustrated solely as a partial blade 142, allowing forobservation of the fin mount 121. The fin mount 121 is bonded within the(missing in this view) inner portion of the propeller blade. In someaspects, the propeller blade is formed from a number of pre-formedpieces which are then bonded together, with the fin mount affixedtherein. The fin mount 121 may be metal, and constructed such that it isadapted to allow for mounting to the main hub 122 with a hinge pin 123,for example. In some embodiments, as seen in FIG. 8A, the fin mount 121may be a plurality of independent pieces. These pieces may be fixturedduring assembly of the propeller blade 108 such that the finishedcomponent is adapted to mount to the main hub 122 with a hinge pin. Astowing tab 143 may be affixed to the fin mount 121 to allow for movingthe blade into a stowed configuration into the recess and against thenacelle body. In some aspects, the propeller blade 108 may be of acomposite material. The propeller blade 108 may be assembled from piecessuch that the blade is a hollow shell assembled from pre-manufacturedindividual pieces. A deploy spring 141 allows for the blades of thepropeller to achieve a deployed configuration in the absence ofcentrifugal forces. The deploy spring allows for full deployment of thepropeller blades even when the rotors are not turning. To achieve fullstowage, the stowing tabs 143 on the propeller blades 108 of thepropeller 107 are pushed on by a stowing mechanism, until the blades arefit within the recesses 110 of the nacelle bodies.

FIG. 9 illustrates another perspective view of a motor driven rotor unitwith even further portions removed for clarity of illustration. The mainhub 122 is seen supporting the fin mount 121. The fin mount 121 isadapted to pivot relative to the main hub 122 using a hinge pin 123. Insome recesses, the partial blades 142 are seen, and other recesses 110no blade is seen, for purpose of visual clarity only. As the furtherportions have been removed for illustrative effect, the rotor deploymentmechanism, the motor, and other components come into view.

FIG. 10 illustrates a side view of portions of a rotor according to someembodiments of the present invention. The propeller blade 108 is seen ina stowed position. The propeller blade 108 is hinged with a hinge pin123 to the main hub 122. The main hub is seen mounted within a bearingassembly 125. The bearing assembly 125 is mounted to the outboardbracket 124 of the rotor deployment mechanism. In some aspects, the mainhub 122 is mounted to the inner race or races of the bearing assembly125, and the outer race of the bearing assembly 125 is mounted withinthe outboard bracket 124 of the rotor deployment mechanism.

FIG. 11 is a side view of portions of a rotor deployment mechanism of adeployable motor driven rotor assembly in a forward flight configurationaccording to some embodiments of the present invention. The mainmounting points 127, 128 are the structural attachment points for therotor deployment mechanism 143, and by extension, for the motor drivenrotor unit, to the aerial vehicle. The drive motor 126 is adapted todrive the rotor main hub 122, and by extension, the propeller of therotor unit. In this forward flight configuration, the rotor thrustvector is oriented facing with regard to the aerial vehicle, and ishorizontal. In some aspects, with the use of rotor deployment mechanismsas described herein, the nacelle may be seen as being split during therotor deployment such that the rear portion of the nacelle stays withthe wing in a fixed positional relationship. The rotor deployment maythen be able to occur from a nacelle along the wing, or along a rearhorizontal stabilizer element. The rotor deployment mechanisms may bemounted at a position that is not the end of the wing, or otherhorizontal element.

FIG. 12 illustrates rotor deployment mechanism 243 in a deployed,vertical take-off, configuration. The rotor deployment mechanism hasboth rotated and displaced the rotor. The deployment has pushed therotor hub forward, and away, from the main mounting points 127, 128, aswell as upward vertically relative to the main mounting points. In thisvertical take-off configuration, the rotor axis is vertical.

The outboard bracket 124 is attached to the deployment linkages at thebracket attach points 134, 135. The bracket arms 129, 130, 131 link viapivot points 132, 133. With the use of multi-arm linkages the rotor maybe moved to preferred positions in both the deployed and stowedconfigurations. FIGS. 13-16 illustrate the rotor with its linkages in apartially deployed configuration, which is seen during transitions fromvertical to horizontal thrusting, or from horizontal to verticalthrusting.

The electric motor/propeller combination being on the outboard side ofthe articulating joint allows for a rigid mounting of the propeller tothe motor, which is maintained even as the propeller is moved throughvarious attitudes relative to the rear nacelle portion. With such aconfiguration the rotating power from the motor need not be gimbaled orotherwise transferred across a rotating joint.

FIG. 17 illustrates a deployment drive system for a deployment mechanismaccording to some embodiments of the present invention. A drive unit 151may be coupled to the aerial vehicle, within the wing in an areaadjacent to the mounting points for the main mounting points 127, 128.Drive screws 150 may be driven such that the deployment linkage isdriven from a stowed configuration to a deployed configuration, and froma deployed configuration to a stowed configuration.

FIG. 18 is a partial view of the underside of a main rotor hub 122mounted into an outboard bracket 124 of a rotor deployment mechanismaccording to some embodiments of the present invention. A stowing rod153 is adapted to drive the stowing levers 152 against the stowing tabs143. The stowing tabs 143 then drive the propeller blades into a nestedposition onto the nacelle body. The deploy springs 141 are adapted todeploy the propeller blades 108 from a stowed position to a deployedposition. FIG. 19 is a partial side cutaway view of the stowing rod 153coupled to a plurality of stowing levers 152. The stowing rod 153 may bedriven by a linear actuator to engage the stowing tabs 143 in order todeploy the propeller blades from their stowed, nested, configuration.When fully deployed, the propeller blades will not reside on the stowinglevers. FIG. 20 is a bottom perspective view of the stowing rod 153 andits coupling to the stowing levers 152, and ultimately to the fin mounts121 of the propeller blades 108. Position indicators may be used toproperly line up the propeller relative to the recesses in the nacelle.

In an exemplary embodiment of a method for flying an aerial vehicle withan articulated electric propulsion system and fully stowing blades, anaerial vehicle may be on the ground. The aerial vehicle may have aplurality of wing and tail mounted motor driven rotor units. The motordriven rotor units may begin with propeller blades that are stowed suchthat the stowed propeller blades comprise all or most of the effectivewetted area of portions of the nacelles of which they form a part. Thenacelles may have recesses adapted to receive the stowed blades.

The stowed blades may be held in the stowed position with the assistanceof stowing mechanisms. In preparation for vertical take-off, the stowedblades may deploy to a deployed configuration. The blades may utilizedeployment springs which assist with the deployment of the blades uponthe release of stowing levers. The stowing levers may be adapted topivot the propeller blades from a deployed to a stowed configuration.

Once the propeller blades are in a deployed position, the entire motordriven rotor assembly may be itself deployed from a forward flightposition to a vertical take-off and landing position with the use of anarticulating rotor deployment mechanism. The deployment mechanism isadapted to position the propellers in front of and above the wings, orotherwise clear of other aerial vehicle structure. With the propellerblades now deployed, and with the motor driven rotor units nowarticulated into a vertical take-off configuration, the aerial vehicleis able to begin a vertical take-off. The rotors are spun up and thevehicle rises from the ground.

After take-off, the aerial vehicle will begin a transition to forwardflight by articulating the rotors from a vertical thrust orientation toa position which includes a horizontal thrust element. As the aerialvehicle begins to move forward with speed, lift will be generated by thewings, thus requiring less vertical thrust form the rotors. As therotors are articulated further towards the forward flight, horizontalthrust, configuration, the aerial vehicle gains more speed.

Once the aerial vehicle is engaged in regular forward flight, the rotorsin use during take-off may no longer be necessary. The thrustrequirement for forward flight may be significantly less than thatrequired during vertical take-off and landing. The forward flight may bemaintained by just a subset of the rotor used for take-off, or bydifferent rotors than those used during take-off. The unused rotors mayhave their propeller blades stowed in to recesses on the nacellessupporting the rotors. The stowed propeller blades may form the exteriorsurface of portions of the nacelle.

As evident from the above description, a wide variety of embodiments maybe configured from the description given herein and additionaladvantages and modifications will readily occur to those skilled in theart. The invention in its broader aspects is, therefore, not limited tothe specific details and illustrative examples shown and described.Accordingly, departures from such details may be made without departingfrom the spirit or scope of the applicant's general invention.

What is claimed is:
 1. A stowable propeller system, said stowablepropeller system comprising: a central hub, said central hub comprisinga first rotation axis; a drive motor, said drive motor rigidly coupledto said central hub; a plurality of propeller blades, said plurality ofpropeller blades pivotally attached to said central hub; and a nacellebody, said nacelle body comprising recesses in its outer surface whensaid propeller blades are in a deployed position, wherein said propellerblades are adapted to stow into said recesses in the outer surface ofthe nacelle body, said blades when stowed comprising all or most of theeffective wetted area of portions of the nacelle.
 2. The stowablepropeller system of claim 1 wherein said stowable propeller systemfurther comprises a first principal axis along its center, and whereinsaid propeller blades are adapted to pivot from a stowed position in therecesses of the nacelle body to a deployed position to a deployedposition substantially perpendicular to said first rotation axis.
 3. Thestowable propeller system of claim 2 further comprising a rotarypositional sensor adapted to sense the rotary position of said propellerblades relative to said nacelle body.
 4. The stowable propeller systemof claim 2 further comprising a stowing mechanism, said stowingmechanism adapted to stow said propeller blades into said recesses inthe outer surface of the nacelle body.
 5. The stowable propeller systemof claim 3 further comprising a stowing mechanism, said stowingmechanism adapted to stow said propeller blades into said recesses inthe outer surface of the nacelle body.
 6. The stowable propeller systemof claim 4 further comprising a spring system adapted to deploy thepropeller blades from their stowed position.
 7. The stowable propellersystem of claim 5 further comprising a spring system adapted to deploythe propeller blades from their stowed position.
 8. The stowablepropeller system of claim 1 further comprising a rotor deploymentmechanism, said rotor deployment mechanism coupled to said central hubon a first end, said rotor deployment mechanism coupled to mountingpoints within said nacelle body on a second end, wherein said rotordeployment mechanism is adapted to deploy said central hub and saiddrive motor.
 9. The stowable propeller system of claim 8 wherein saidnacelle body comprises a first principal axis along its center, andwherein said rotor deployment mechanism comprises one or more linkageassemblies adapted to deploy said central hub and said drive motor froma first position wherein said first rotation axis and said firstprincipal axis are parallel to a second position wherein said firstrotation axis and said first principal axis are substantiallyperpendicular.
 10. The stowable propeller system of claim 6 furthercomprising a rotor deployment mechanism, said rotor deployment mechanismcoupled to said central hub on a first end, said rotor deploymentmechanism coupled to mounting points within said nacelle body on asecond end, wherein said rotor deployment mechanism is adapted to deploysaid central hub and said drive motor.
 11. The stowable propeller systemof claim 10 wherein said nacelle body comprises a first principal axisalong its center, and wherein said rotor deployment mechanism comprisesone or more linkage assemblies adapted to deploy said central hub andsaid drive motor from a first position wherein said first rotation axisand said first principal axis are parallel to a second position whereinsaid first rotation axis and said first principal axis are substantiallyperpendicular.
 12. The stowable propeller system of claim 8 wherein saidnacelle body comprises a first principal axis along its center, andwherein said rotor deployment mechanism comprises one or more linkageassemblies adapted to deploy said central hub from a first positionwherein said first rotation axis and said first principal axis areparallel to a second position wherein said first rotation axis and saidfirst principal axis are perpendicular, and further wherein thedeployment of the central hub deploys said central hub away from saidmain rotor mounting nacelle body, wherein the deployment of the centralhub deploys said central hub away from said first principal axis. 13.The stowable propeller system of claim 10 wherein said nacelle bodycomprises a first principal axis along its center, and wherein saidpropeller deployment mechanism comprises one or more linkage assembliesadapted to deploy said central hub from a first position wherein saidfirst rotation axis and said first principal axis are parallel to asecond position wherein said first rotation axis and said firstprincipal axis are perpendicular, and further wherein the deployment ofthe central hub deploys said central hub away from said nacelle body,wherein the deployment of the central hub deploys said central hub awayfrom said first principal axis.
 14. A stowable propeller system, saidstowable propeller system comprising: a first propeller portion, saidfirst propeller portion comprising: a central hub, said central hubcomprising a first rotation axis; a drive motor, said drive motorrigidly coupled to said central hub; a first plurality of propellerblades, said first plurality of propeller blades pivotally attached tosaid central hub; a main rotor mounting nacelle, said main rotormounting nacelle comprising: a first principal axis along the center ofsaid main rotor mounting nacelle; a hub support coupled to said centralhub; a nacelle body, said nacelle body comprising recesses in its outersurface when said first plurality of propeller blades are in a deployedposition, wherein said first plurality of propeller blades are adaptedto stow into said recesses in the outer surface of the nacelle body,said first plurality of propeller blades when stowed comprising all ormost of the effective wetted area of portions of the nacelle, and asecond propeller portion, said second propeller portion comprising: asecond plurality of propeller blades, said second plurality of propellerblades coupled to said central hub.
 15. The stowable propeller system ofclaim 14 wherein said first plurality of propeller blades are adapted topivot from a stowed position in the recesses of the nacelle body to adeployed position to a deployed position substantially perpendicular tosaid first rotation axis.
 16. The stowable propeller system of claim 15further comprising a stowing mechanism, said stowing mechanism adaptedto stow said first plurality of propeller blades into said recesses inthe outer surface of the nacelle body.
 17. The stowable propeller systemof claim 1 wherein the blades of said plurality of propeller blades areforward swept blades.
 18. The stowable propeller system of claim 8wherein the blades of said plurality of propeller blades are forwardswept blades.
 19. The stowable propeller system of claim 12 wherein theblades of said plurality of propeller blades are forward swept blades.20. The stowable propeller system of claim 14 wherein the blades of saidfirst plurality of propeller blades are forward swept blades.